METHOD AND DEVICE FOR THE EFFICIENT REDUCTION OF CARBON DIOXIDE EMISSIONS

Description

[0001] The invention relates to a method for safely binding carbon dioxide with old building materials, for example cement stone, and thus ensuring safe long-term storage.

[0002] It is becoming increasingly necessary to conserve natural resources and utilize recycled materials. Carbon dioxide emissions are also a critical cause of global warming. Therefore, there is a growing focus on capturing carbon dioxide from exhaust gases and storing or utilizing it permanently. One possible method is injecting liquefied carbon dioxide into the ground. However, this method is not without controversy, as its long-term retention is not guaranteed, and any escape would exacerbate the greenhouse effect. Furthermore, capturing and storing the carbon dioxide requires additional energy, potentially producing more carbon dioxide.

[0003] One of the most carbon-intensive industries is the cement industry. Firstly, the process requires a great deal of energy, which, when using conventional fossil fuels, leads to carbon dioxide emissions. Secondly, carbon dioxide is released from the raw material, such as limestone, as a result of the process itself.

[0004] On the other hand, large quantities of old concrete are generated when concrete structures are demolished. Therefore, the recycling of concrete, for example to produce new cement, is currently being discussed. However, a problem arises because components such as sand and hardened cement are often difficult to separate and are inextricably mixed and bonded together. The sand-free or at least low-sand component of old concrete is also referred to as recycled cement. It is known that concrete can absorb carbon dioxide during its service life, but only a fraction of the carbon dioxide released from the limestone during its production. After a long period, for example in very old buildings, this value can be around 20% relative to the calcium content of the concrete; thus, approximately one-fifth of the originally released carbon dioxide is reabsorbed very slowly and over long periods.

[0005] From WO 2020 / 058 247 A1 a process and a plant for processing material containing cement stone are known.

[0006] The use of carbon dioxide from and for cement is known from EP 3 656 750 A2.

[0007] A method for reducing carbon dioxide emissions is known from US 2001 / 054253 A1.

[0008] From FR 3 113 465 A1, a forced carbonization process of a fine fraction of demolition concrete is known.

[0009] A material and a method for carbon dioxide sequestration are known from US patent 2010 / 0068109 A1.

[0010] The object of the invention is to provide a safe and durable method for the separation and final storage of carbon dioxide without requiring a complex, especially energy-intensive process that itself represents a new source of emissions.

[0011] This problem is solved by the method with the features specified in claim 1 and by the carbon dioxide separation plant with the features specified in claim 13. Advantageous further developments are described in the dependent claims, the following description, and the drawings.

[0012] The process according to the invention serves to bind carbon dioxide to a waste building material. The waste building material is waste concrete or a product made from waste concrete, for example, waste cement brick. Waste cement brick is the component depleted of the sand / gravel component, which therefore has a higher proportion of calcium compounds. The process comprises the following steps, carried out sequentially: a) Providing the waste building material, b) Moistening the waste building material to a moisture content of 5 to 25 wt.%, c) Introducing the waste building material into a carbon dioxide-containing gas stream and transporting the waste building material in the gas stream for at least 1 s in a fluidized bed reactor, d) Separating the waste building material from the gas stream.

[0013] The steps are carried out sequentially, focusing on the individual particles of the old building material. Since this is a continuous process, all process steps are performed simultaneously, but along the process chain.

[0014] This process differs fundamentally from previous methods. The treatment takes place during transport within the gas stream. Therefore, it is not carried out in an aqueous environment as before, but rather in a comparatively semi-dry environment within the gas stream.

[0015] A key aspect of this process is that it takes place in an entrained flow reactor. This distinguishes it significantly from a fluidized bed, as shown, for example, in US 2001 / 005453 A1, or a slurry sinking against the gas flow, as shown in US 2010 / 0068109 A1. Two parameters are particularly important for transport in the gas flow of an entrained flow reactor. First, the loading is on the order of 1 kg of solid per kg of gas or significantly less. Second, the particle size must be small enough, which means that the particles must be separated and not present as a slurry in order to be transported by the gas flow in an entrained flow reactor. This also requires that the airborne particles not be agglomerated by water to form larger agglomerates.

[0016] This makes the process particularly gentle. The reaction occurs directly from the gas phase, which contains carbon dioxide, with the waste material. This allows for very rapid conversion, even for relatively small particles (enough to be airborne). Simultaneously, transport in the gas phase facilitates the separation of even the smallest particles, making it easy to implement different residence times for different particle sizes. Since diffusion within the particle is the rate-determining step, the smaller the particles, the faster the conversion process. Consequently, a shorter residence time is required. Therefore, using an entrained flow reactor offers advantages over other reactor types.It is essential that the old building material is dry enough to be transported in a gas stream, yet moist enough for the reaction to proceed quickly. Therefore, a "semi-dry" formulation is suitable. Ideally, a thin film of water can form on the particle surface, or water can penetrate the particle pores. At the same time, the formation of sludge, which would prevent it from being transported and treated in the gas stream, must be prevented. This optimum is achieved for old building materials, particularly old cement bricks, at a water content (moisture) of 5 to 25% by weight. This moisture refers only to volatile water, not water of crystallization or water otherwise bound within the crystal structure. Only this free water on the surface is available for the actual reaction.Therefore, adjusting the humidity to the relatively narrow range of 5 to 25 wt% is essential, as this ensures flight capability.

[0017] The moisture content can be determined using a conventional moisture meter, for example at 80 °C. This ensures that only the moisture on the surface, including the pores, is measured, while water of crystallization is not included.

[0018] The combination according to the invention of a moisture content, i.e. free water on the surface, of 5 to 25 wt.% and the reaction in the airflow leads to a particularly simple and fast conversion.

[0019] Old building material within the meaning of the invention includes in particular construction waste, broken concrete, cement stone and the like in various stages of processing.

[0020] In a further embodiment of the invention, comminution takes place before or after the introduction in step c). The comminution particularly includes deagglomeration of adhering particles. In particular, the moistening in step b) can cause particles to adhere to one another.

[0021] For example, the transport and thus the conversion in step c) takes place in a riser pipe. A hammer mill is located below this riser pipe. In this embodiment, the waste building material can, for example, be introduced directly into the hammer mill. Alternatively, the waste building material can be introduced into the riser pipe. In this case, particles that are too large and not carried by the gas flow sink down, enter the hammer mill, are crushed there, and are then picked up and transported by the gas flow. This crushing leads to two positive effects. Firstly, large particles become smaller, so that they can be transported more easily in a gas flow. Secondly, the smaller particle size reduces the diffusion length for the carbon dioxide and thus shortens the reaction time.

[0022] In a further embodiment of the invention, additional humidification takes place during transport in step c) in addition to the humidification in step b). This can be achieved, for example, by spraying water. Alternatively or additionally, steam can also be used. This prevents excessive drying during transport in the gas stream. In particular, this allows the moisture content in step b) to be selected to be lower, thus enabling improved transport in the gas stream in step c). Furthermore, this compensates for drying out, ensuring that the moisture content always remains within the optimal range.

[0023] In a further embodiment of the invention, the separation in step d) is carried out in two stages. First, in a first step, the larger particles are separated, and then in a second step, the smaller particles are separated. The first step can, for example, take place in a separation cyclone or a classifier. Subsequently, the gas stream is passed through a filter. In the separation cyclone or classifier, the larger particles are separated first, and in the filter, the smaller particles. The first and second steps can be carried out simultaneously. Alternatively, the first and second steps can be carried out sequentially, with either the first step occurring before the second step or the second step occurring before the first step.

[0024] In a further embodiment of the invention, the waste building material is recycled after step d), so that steps b) to d) are repeated. This allows the carbon dioxide loading capacity of the waste building material, which typically corresponds to approximately 80% to 90% of the amount of, for example, CaO, to be fully utilized. If the separation is carried out in two stages, resulting in two different fractions of the waste building material, then, for example, only the fraction from the first separation step, i.e., the larger particles, can be recycled. In this case, the finer particles separated in the second separation step are removed. This is preferred if the fraction of the smallest particles has already been completely loaded with carbon dioxide in a single pass (within the technically feasible range, i.e., in particular in the range of approximately 80% to 95% of the theoretically maximum possible conversion).Alternatively, the second faction can also be partially or completely withdrawn.

[0025] In a further embodiment of the invention, the CO₂ content of the recycled building material is determined. The recycled material is recycled, and steps b) to d) are repeated until a predetermined CO₂ content threshold is exceeded. Typically, a maximum loading of approximately 95% is assumed, based, for example, on the theoretical CaO content of the recycled building material. This corresponds to the amount that concrete is capable of absorbing under normal circumstances. The threshold can therefore be set, for example, at 60%, 80%, 85%, 90%, or 95%, so that the absorption capacity of the recycled building material is utilized as fully as possible without entering a kinetically unfavorable range. If the threshold is exceeded, the recycled building material is removed, and preferably new recycled building material is added.While naturally only a maximum of 20% of carbon dioxide can be rebound in the concrete, the invention permanently rebinds significantly more carbon dioxide.

[0026] In a further embodiment of the invention, the threshold value of the CO₂ content is set at 60%, preferably 70%, more preferably 80%, more preferably 85%, more preferably 90%, and more preferably 85% of the sum of calcium, magnesium, and iron. Here, for example, the amount of iron can also be 0 if the old building material contains no iron. As already explained, the limit of approximately 20% is easily achievable within the building itself, without further assistance, through carbon dioxide loading over a very long period, so that the supplied old building material has a loading between 0% and 20%. The threshold value is intended to prevent an insufficient residual absorption capacity (relative to the hypothetically / calculatively possible 100%) from resulting in practically no further carbon dioxide being absorbed from the gas stream.The threshold value thus ensures a balance between maximum utilization of the absorption capacity of the old building material and the kinetics of carbon dioxide capture.

[0027] In a further embodiment of the invention, the separation in step d) is size-selective, resulting in at least two fractions. This can be achieved, for example, as already described, using a separation cyclone or classifier and a filter. The coarser fraction is recycled, and the finer fraction is removed from the process. Due to the significantly shorter diffusion paths for carbon dioxide from the gas stream, the reaction in small particles is considerably faster than in large particles. Therefore, removing very fine particles, for example after separation from a filter, is particularly useful and effective.

[0028] In a further embodiment of the invention, after step d), the gas stream is at least partially recirculated before step c). This allows the carbon dioxide content of the gas stream to be reduced. For example, the gas stream can be completely recirculated until a certain predetermined carbon dioxide concentration is reached. This has the advantage that particularly low carbon dioxide concentrations can be achieved, but the disadvantage that the treatment of the gas stream is only discontinuous, which is unfavorable for most exhaust gas-producing plants. Alternatively, only partial recirculation is performed. This allows for a lower carbon dioxide concentration than with a simple pass-through. At the same time, a continuous supply and discharge of gas is possible, which simplifies the treatment of an exhaust gas stream. However, it should be noted that the gas stream processed by the method increases to the extent of the recirculation.For example, if 80% of the gas flow is recycled, the gas flow in step c) is five times larger than the supplied exhaust gas flow to be cleaned.

[0029] In a further embodiment of the invention, the carbon dioxide concentration in the gas stream is measured after transport in step c). The proportion of the gas stream recycled before step c) is selected depending on the measured carbon dioxide concentration. This active control enables, for example, compliance with emission limits. Additionally, it can also be used to detect when the material can no longer absorb carbon dioxide, i.e., when its carbon dioxide absorption capacity is exhausted after the reaction has taken place.

[0030] In a further embodiment of the invention, the moisture content is measured before and / or after humidification in step b). The moisture content is preferably measured using a NIR sensor. Preferably, the humidification process in step b) is controlled based on the measured moisture content. It is important to keep the moisture content high enough to ensure efficient carbon dioxide uptake. On the other hand, the moisture content should remain low enough to allow transport in the gas stream. Therefore, active control is advantageous.

[0031] In a further embodiment of the invention, an exhaust gas stream is selected as the gas stream in step c). For example, the exhaust gas stream can originate from a cement plant. For instance, the exhaust gas stream has a temperature between 80 °C and 200 °C. Preferably, the exhaust gas stream is not heated. The advantage of this method is therefore the low energy consumption for the reliable and long-term binding of carbon dioxide from the exhaust gas. However, an exhaust gas stream from, for example, a waste incineration plant can also be used.

[0032] In another aspect, the invention relates to a carbon dioxide separation system. In particular, the carbon dioxide separation system is designed to carry out the process according to the invention. The carbon dioxide separation system comprises a gas inlet, a treatment device downstream of the gas inlet, a separation device downstream of the treatment device, and a gas outlet downstream of the separation device. Furthermore, the carbon dioxide separation system comprises a waste building material inlet. The waste building material inlet is connected to a humidification device for transferring the waste building material. The connection can be realized either directly or, for example, via an intermediate waste building material storage unit. The waste building material is humidified in the humidification device. The humidification device is connected to the treatment device.In the treatment device, the moistened waste material stream thus meets the gas stream. Here, carbon dioxide transfers from the gas stream into the waste material stream. The treatment device is an entrained-flow reactor, specifically a riser pipe. The separation device is connected to a waste material outlet. The waste material, now laden with carbon dioxide, can be removed via the waste material outlet and, for example, disposed of in a final storage facility or used for further processing.

[0033] The humidification device can be, for example, a humidification screw or a mixer.

[0034] Thus, from the treatment unit to the separation unit, the gas flow and the waste building material flow are guided in parallel, with the waste building material flowing within the gas flow. This distinguishes the device from conventional systems that operate with a liquid-solid phase in the treatment unit, meaning that separation occurs directly within the treatment unit, eliminating the need for a downstream separation unit. This also means that significantly less water is required, resulting in a much drier product (waste building material with bound carbon dioxide) that can be used without further energy input, for example, for long-term storage, such as reuse in the construction industry.

[0035] In a further embodiment of the invention, the carbon dioxide separation system includes a first waste building material storage unit. This storage unit is arranged between the waste building material feed and the humidification device. The waste building material storage unit is particularly advantageous when the waste building material is recycled.

[0036] In a further embodiment of the invention, the carbon dioxide separation system includes a second waste building material storage tank. The second waste building material storage tank is preferably arranged parallel to the first. Thus, waste building material can be supplied to the humidification device from either the first or the second waste building material storage tank. Likewise, waste building material can be supplied from the waste building material feed to either the first or the second waste building material storage tank. If a return system is available, the return material can also be supplied to either the first or the second waste building material storage tank. It is preferred that, for example, waste building material is supplied to the humidification device from the first waste building material storage tank and then returned to the first waste building material storage tank via the return system.Simultaneously, the second waste building material storage tank can be filled, for example, from the waste building material inlet. Once the waste building material, which is circulated in a loop, is sufficiently saturated with carbon dioxide, it can be removed from the carbon dioxide separation unit. Subsequently, waste building material is fed into the humidification unit of the second waste building material storage tank and then returned to the second storage tank via the return line. The now-empty first waste building material storage tank can then be refilled via the waste building material inlet.

[0037] In a further embodiment of the invention, the carbon dioxide separation system includes a recirculation device. The recirculation device is connected to the separation device for receiving waste building material from the separation device. The recirculation device is designed to return the waste building material to the humidification device, in particular to the first waste building material storage tank and / or an optional second waste building material storage tank. This recirculation enables optimal loading and thus efficient utilization of the waste building material for the separation and binding of carbon dioxide. It also makes it possible to use a very broad particle size distribution, which consequently exhibits very different kinetics.

[0038] The use of a recycling device with a first and a second waste building material storage tank enables a further variation of the process. For example, fresh waste building material from the waste building material feed is fed only into the first waste building material storage tank, and the return from the recycling device is fed only into the second waste building material storage tank. In this case, only a portion of the waste building material is recycled after separation. Based on the proportion removed, waste building material is fed to the humidification device from both the first and second waste building material storage tanks. If, for example, 80% of the waste building material is recycled, then 1 / 5 of the waste building material fed to the humidification device is taken from the first waste building material storage tank and 4 / 5 from the second waste building material storage tank.

[0039] In a further embodiment of the invention, the separation device comprises a first partial separation device and a second partial separation device. In particular, the first partial separation device is a classifier or a separation cyclone, and the second partial separation device is a filter. This has two advantages. In addition to separating a very fine fraction that is typically already saturated with carbon dioxide after the first pass, the exhaust gas stream is also sufficiently freed of old building material to be released into the environment.

[0040] In a further embodiment of the invention, the first partial separation device is connected to the return device. The second partial separation device is connected to the waste material outlet. In this embodiment, the fine fraction is thus directly discharged, while the coarse fraction is recycled. An additional advantage is that the cycle is not burdened by the fine fraction, which is already saturated with carbon dioxide.

[0041] In a further embodiment of the invention, the treatment device includes a post-humidification device. The post-humidification device can, for example, be a water injection system or the supply of steam. This prevents the old building material from drying out too much in the treatment device and simultaneously allows a very low moisture content to be set in the humidification device, which in turn results in very good airability of the old building material.

[0042] In another embodiment of the invention, the humidification device is a humidification screw.

[0043] In a further embodiment of the invention, the carbon dioxide separation system includes a comminution device. The comminution device is arranged upstream of the treatment device in terms of gas flow. For example, and preferably, the comminution device is a hammer mill. Another exemplary and preferably configuration is that the treatment device is designed as a riser pipe and the comminution device is arranged below the treatment device. This arrangement has the advantage that, in particular, large particles, which are not carried upwards in the riser pipe by the gas flow but instead fall downwards into the comminution device by gravity, are comminuted there and thus ultimately become airborne, and are then carried out of the treatment device by the gas flow. The advantage is that even small, airborne particles are not further comminuted, thereby saving energy.Of course, it is also possible to feed the entire waste material from the humidification unit into the shredding unit. This is particularly advantageous if, in particular, the fine fraction is removed from the cycle after a filter, while the larger fraction is recycled after a separation cyclone or classifier. The continuous reintroduction into the shredding unit thus increases the proportion of fine material discharged.

[0044] In a further embodiment of the invention, the comminution device is arranged below the treatment device. As already explained, this is preferred when the treatment device is designed as a riser pipe.

[0045] In another embodiment of the invention, the treatment device is designed as a riser pipe.

[0046] In another embodiment of the invention, the comminution device is a hammer mill.

[0047] In a further embodiment of the invention, the carbon dioxide separation system features gas recirculation. The gas recirculation is designed to return at least a partial gas stream from the separation device to the treatment device. This enables a particularly high reduction of carbon dioxide in the gas stream.

[0048] In a further embodiment of the invention, the carbon dioxide separation system includes a sensor for measuring the carbon dioxide concentration in the gas stream. The sensor is arranged downstream of the treatment device to measure the carbon dioxide concentration. This makes it possible either to cyclically discharge the gas, which has been sufficiently freed of carbon dioxide, completely or to adjust the ratio between recirculation and discharge, for example, to comply with a maximum permissible emission limit.

[0049] The carbon dioxide separation system according to the invention is explained in more detail below with reference to an embodiment shown in the drawings. Fig. 1 first embodiment Fig. 2 second embodiment Fig. 3 third embodiment

[0050] In Fig. 1 A first embodiment is shown. Waste material, for example cement paste, is introduced into the system via a waste material feeder 10. For example, the waste material is first introduced into the first waste material storage tank 21. From the first waste material storage tank 21, the waste material is conveyed to the humidification device 30, passing a moisture sensor 32. Water is added to the humidification device 30 according to the moisture content of the waste material detected by the moisture sensor 32, in order to achieve a water content of, for example, 15% by weight. For example, the humidification device 30 is designed as a humidification screw. This allows for rapid and thorough mixing. From the humidification device 30, the waste material is introduced into the treatment device 40, which is in the form of a riser pipe. Exhaust gas from a gas supply 80 flows through the treatment device 40, which is in the form of a riser pipe.The gas flow carries the small particles of the waste building material upwards, while larger particles sink to the bottom. A comminution device 42 in the form of a hammer mill is located below the treatment device 40. There, the large particles are crushed until they are carried away by the gas flow and transferred to the treatment device 40. In the treatment device 40, a reaction occurs between the carbon dioxide from the gas flow and, for example, CaO from the waste building material, forming CaCO₃. This process removes the carbon dioxide from the gas flow and binds it permanently. In a subsequent separation cyclone 44, the majority of the waste building material is removed from the gas flow. The gas flow is then passed through a filter for further purification, in which even very fine waste building material particles are separated. Finally, a sensor 52 measures the carbon dioxide concentration in the gas flow.Depending on the measured carbon dioxide concentration, the gas flow divider 54 is regulated. There are three fundamentally different approaches. First, the gas flow divider 54 can completely recirculate the gas flow until the measured carbon dioxide concentration falls below a preset value (e.g., <2 vol%). Then, the gas flow divider 54 is switched, and the gas flow is completely directed to the gas outlet 56, for example, a chimney. Simultaneously, new gas is supplied from the gas inlet 80. Second, the gas flow divider 54 can be variably regulated, whereby the amount directed to the gas outlet 56 is increased as the measured carbon dioxide concentration decreases. In a third variant, the sensor for measuring the carbon dioxide concentration 52 and the gas flow divider 54 can be omitted; the gas flow is directed completely to the gas outlet 56.This method is preferred when only a small portion of the carbon dioxide needs to be removed, for example, to purify only the carbon dioxide component from the exhaust gas in a cement plant powered entirely by renewable energy sources, specifically that originates from the mineral feedstocks. However, since sufficient waste material is likely unavailable for complete carbon dioxide removal in such an application, and therefore only partial separation is possible, direct discharge of the gas stream can be advantageous.

[0051] The waste material from the separation cyclone 44 and the filter 50 is combined and fed to a waste material flow divider 70. This divider can also be operated in different ways. Either the waste material is recycled cyclically and only temporarily discharged, preferably completely, for example, when the carbonate content in the waste material is so high that further recycling is not practical. Alternatively, a partial stream can be continuously separated and discharged, while a second partial stream is recycled. This second partial stream is, for example, fed to the second waste material storage tank.

[0052] Fig. 2 Figure 1 shows a second embodiment. This differs from the first embodiment in that the waste building material separated from the filter 50 is directly discharged and not recycled. Secondly, the system includes a carbonate sensor 72, which measures the carbonate content in the waste building material after separation in the separation cyclone 44. Preferably, the waste building material flow divider 70 returns the entire waste building material flow until the carbonate content measured by the carbonate sensor 72 exceeds a threshold value. At that point, the entire waste building material flow is discharged. Simultaneously, the supply of waste building material from the first and second waste building material storage tanks is switched, thus reversing their function.As a third difference, the old building material is introduced directly into the shredding device 42 after the humidification device 30, thus increasing the proportion of small and therefore very fast reacting particles and at the same time increasing the discharge of the old building material after separation in the filter 50.

[0053] In Fig. 3 A third embodiment is shown. This third embodiment differs from the second embodiment in that a classifier 43 is arranged at the end of the treatment device 40, which is designed as a riser pipe. Very coarse particles of the waste building material are separated there and completely recycled. The waste building material separated in the separation cyclone 44 and the filter 50 is removed from the system. Reference sign

[0054] 10 Waste material feed 21 First waste material storage 22 Second waste material storage 30 Humidification device 32 Humidity sensor 40 Treatment device 42 Shredding device 43 Sifter 44 Separation cyclone 50 Filter 52 Carbon dioxide concentration sensor 54 Gas flow divider 56 Gas outlet 70 Waste material flow divider 72 Carbonate sensor 80 Gas feed

Claims

1. A method for binding carbon dioxide to an old building material, the old building material being old concrete or a product of old concrete, the method comprising the following successive steps: a) Provision of the old building material, b) Moisturising the old building material to a moisture content of 5 to 25% by weight, whereby the moisture only comprises volatile water, i.e. no water of crystallisation or water otherwise incorporated in the crystal structure, c) Introduction of the used building material into a carbon dioxide-containing gas stream and transport of the used building material in the gas stream for at least 1 s in an entrained-flow reactor, d) Separation of the old building material from the gas flow.

2. Method according to claim 1, characterised in that comminution takes place before or after the introduction in step c).

3. Method according to one of the preceding claims, characterised in that during the transport in step c) a remoistening takes place in addition to the moistening in step b).

4. Process according to one of the preceding claims, characterised in that the separation in step d) is carried out in two stages and initially the larger particles are separated in a first sub-step and the smaller particles are separated in a second sub-step.

5. Method according to one of the preceding claims, characterised in that after step d) the old building material is recycled, so that steps b) to d) are repeated.

6. Method according to claim 5, characterised in that the CO2 content of the old building material is determined, the old building material being recycled and steps b) to d) being repeated until a predetermined threshold value of the CO2 content is exceeded.

7. Method according to claim 6, characterised in that the threshold value of the CO2 content is set at 60 %, preferably 70 %, more preferably 80 %, more preferably 85 %, more preferably 90 %, more preferably 85 %,of the sum of calcium, magnesium and iron.

8. Process according to one of claims 5 to 7, characterised in that the separation in step d) is carried out size-selectively in at least two fractions, the coarser fraction being recycled and the finer fraction being discharged from the process.

9. Method according to one of the preceding claims, characterised in that a measurement of the moisture content is carried out before and / or after the moistening in step b), the measurement of the moisture content preferably being carried out by means of an NIR sensor.

10. Method according to one of the preceding claims, characterised in that an exhaust gas stream is selected as the gas stream in step c).

11. Process according to one of the preceding claims, characterised in that after step d) the gas stream is at least partially recycled before step c).

12. Method according to claim 11, characterised in that the carbon dioxide concentration in the gas stream after transport in step c) is detected, the proportion of the quantity of the gas stream returned before step c) being selected as a function of the carbon dioxide concentration determined13. Carbon dioxide separation plant, the carbon dioxide separation plant having a gas feed (80), a treatment device (40) downstream of the gas feed (80), a separation device downstream of the treatment device (40) and a gas outlet (56) downstream of the separation device, wherein the carbon dioxide separation plant has an old building material feed (10), wherein the old building material feed (10) is connected to a humidification device (30) for transferring the old building material, wherein the humidification device (30) is connected to the treatment device (40), wherein the treatment device (40) is an entrained-flow reactor, wherein the separation device is connected to an old building material outlet.

14. Carbon dioxide separation plant according to claim 13, characterised in that the carbon dioxide separation plant has a first used building material store (21), the first used building material store (21) being arranged between the used building material feed (10) and the humidification device (30).

15. Carbon dioxide separation plant according to one of claims 13 to 14, characterised in that the carbon dioxide separation plant has a return device, the return device being connected to the separation device for receiving old building material from the separation device, the return device being designed to return the old building material upstream of the humidification device (30), in particular into the first old building material store (21).

16. A carbon dioxide separation system according to any one of claims 13 to 15, characterised in that the separation device comprises a first partial separation device and a second partial separation device.

17. Carbon dioxide separation plant according to claim 16, characterised in that the first partial separation device is a separator (43) or a separator cyclone (44), the second partial separation device being a filter (50).

18. Carbon dioxide separation plant according to one of claims 16 to 17, characterised in that the first partial separation device is connected to the recirculation device and the second partial separation device is connected to the waste material outlet.

19. Carbon dioxide separation plant according to one of claims 13 to 18, characterised in that the treatment device (40) comprises a post-humidification device.

20. Carbon dioxide separation plant according to one of claims 13 to 19, characterised in that the humidification device (30) is a humidification screw.

21. Carbon dioxide separation plant according to one of claims 13 to 20, characterised in that the carbon dioxide separation plant has a comminution device (42), the comminution device (42) being arranged upstream of the treatment device (40) in terms of gas flow.

22. Carbon dioxide separation plant according to claim 21, characterised in that the comminution device (42) is arranged below the treatment device (40).

23. Carbon dioxide separation plant according to claim 22, characterised in that the treatment device (40) is designed as a riser pipe.

24. Carbon dioxide separation plant according to any one of claims 21 to 23, characterised in that the comminution device (42) is a hammer mill.

25. Carbon dioxide separation plant according to one of claims 13 to 24, characterised in that the carbon dioxide separation plant has a gas recirculation system, the gas recirculation system being designed to recirculate at least one partial gas stream from the separation device to the treatment device (40).

26. Carbon dioxide separation plant according to claim 25, characterised in that the carbon dioxide separation plant has a sensor for measuring the carbon dioxide concentration (52) in the gas stream, the sensor for measuring the carbon dioxide concentration (52) being arranged downstream of the treatment device (40).